Conventionally it has long been accepted that coated metal sheets, such as coated steel sheets, can not be applied to electric resistance weldings, such as spot welding and seam welding unless for special welding methods and special paints are used, because the coating films are electrically insulating.
As for weldable paints, paints such as zinc-rich paints which contain a large amount of metallic zinc have been known, but these paints do not give satisfactory electric conductivity unless 90% or more of the zinc metal is contained, and due to the large content of the metal, these paints have poor workability and thus have been limited in their applications.
In efforts for solving the difficulties confronted by with the above conventional paints, a method has been proposed in U.S. Pat. No. 3,904,555. According to this method, a mixture of soft metals and hard metals is added to the paint composition and this method has succeeded in lowering the amount of the metal addition.
However, the coated steel sheet according to U.S. Pat. No. 3,904,555 has various defects such that the metal powders themselves contained in the coating layer rust, that the metal powders form a local cell with the steel sheet substrate causing earlier corrosion of the steel sheet substrate, that the metal powders contained in the coating layer react with the welding electrode to form an alloy of the powder metal and electrode metal on the electrode surface, and that the metal powders are oxidized and edhere to the welding electrode, thus causing consumption of the electrode.